The present invention relates to a roller conveyor for conveying individual items, particularly pallets, and for permitting such items to accumulate at the end of a conveying path without being subjected to pressure.
Conveyors of this type generally include supporting rollers to support the items, each of these rollers being driven by an intermediate roller equipped with a sprocket wheel which engages an endless chain. The intermediate roller can be placed into either a driving position or an idling position by a rod assembly which is provided with sensors extending into the path of the items.
German patent application No. P 25 17 015.4 discloses a conveyor for conveying and accumulating individual items, particularly pallets, in which the intermediate roller is provided with a sprocket wheel meshing with an endless chain. The drive wheel which can be brought into a driving connection with the supporting roller may also be designed as a sprocket wheel. This produces a doubly positively locking drive transmission which, on the one hand, assures in an advantageous manner safe and smooth movement of the items but, on the other hand, has the drawback that it continues to run even when an item is blocked so that there is a large waste of energy and the conveyor and/or the items may be damaged.
Difficulties also arise during coupling when the sprockets are brought into engagement. If engagement has been established, the supporting rollers start rotating with a jerk at the desired speed. This produces high load peaks in the drive system and jerky movements of the items so that lateral escape from the path, with items becoming jammed and falling off, is made possible.
The drive wheel may also be a friction wheel, in which case the establishment and release of the driving connection presents no problems. The drawback of this form of construction, however, is that the driving wheel continues to rotate when the supporting roller is blocked, resulting, due to the strong friction and the resulting heating of the rubber-like friction coating, in wear and destruction and consequently loss or reduction of drive output.
Since, in warehouse areas, the conveyor paths are arranged in close juxtaposition without provision of spaces therebetween, and at several levels on top of one another, it is difficult and expensive to control the numerous drive connections or to locate and replace a damaged part. Furthermore, the coefficient of friction of such a drive connection is greatly reduced by humidity and dust so that such conditions can cause movement of the items to be slowed down and even completely interrupted.
The fact that items often rest only partly on a single driven support roller hinders rapid acceleration of the items from a standstill. If the items are in insufficient contact with that supporting roller or if one of the above-described reductions in drive output exists, the items will not start moving. This results in a malfunction of the system with possible damage thereto.
Another known system is composed of accumulating roller conveyors with directly driven supporting rollers in which a slide coupling is provided between the driven wheel of the support roller and the roller jacket, which coupling slides through when items accumulate. Since the supporting rollers are being driven continuously, the coupling friction must also be continuously overcome. The larger the number of accumulated items, the greater is the amount of energy wasted. Moreover, the first item on each conveyor is also subjected to considerable accumulation pressures exerted by the subsequent items so that pressure damage may occur.
The known roller conveyors thus exhibit drawbacks which do not assure trouble-free continuous operation.